<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      .sidebar {
        grid-area: sidebar;
      }

      .content {
        grid-area: content;
      }

      .header {
        grid-area: header;
      }

      .wrapper {
        margin: 50px;
        display: grid;
        grid-gap: 10px;
        grid-template-columns: 120px 120px 120px;
        grid-template-areas:
          ". header  header"
          "sidebar content content";
        background-color: #fff;
        color: #444;
      }
      .box {
        background-color: #444;
        color: #fff;
        border-radius: 5px;
        padding: 20px;
        font-size: 150%;
      }

      .header {
        background-color: #999;
      }
    </style>
  </head>
  <body>
    <div class="wrapper">
      <div class="box header">Header</div>
      <div class="box sidebar">Sidebar</div>
      <div class="box content">Content</div>
    </div>
  </body>
</html>
